Electropolishing of molybdenum



, A ril 1, 1958 E. R. BOWERMAN E AL 2,829,097

I ELECTROPOLISHING OF MOLYBDENUM Filed Sept. 13, 1954 2 Sheets-Sheet 2 FIG. 2. I

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/o HYDROCHLOR IC AC ID IN VEN TOR. EDWA 5. 640558 ma m 47'TORNEY tates 2,8293% ELECTRGPQLISL HNG F MGLYBDENUM Edwin Bowcrrnan, Whitestone, and Edward B.

baubestre, Elmhurst, N. Y., assignors to Sylvania Electric Products End, a corporation of Massachusetts Application epternber 13, 1954, Serial No. 455,503 9 Qiaims. (Cl. Zti4--14ti.5)

The present invention relates to electrolytic treatment of metals, and in particular to improved methods and baths for electropolishing molybdenum. v

Metals such as molybdenum find widespread application in the fabrication of electronic components. For

example, in the manufacture of vacuumtubes, wound grids of molybdenum are used in that high dimensional stability is required for the relatively high operating temperatures of the tube. Numerous other applications are found for dimensionally stable molybdenum, for example, as in the unipotential cathode assembly of a magnetron which assembly likewise encounters a high operating temperature. The above as well as many other electronic devices require close dimensional tolerances, and components in critically spaced relation to obtain desirable characteristics.

As a practical matter, molybdenum is relatively hard to machine. Molybdenum, although softer and more ductile than tungsten, is exceptionally valuable in the manufacture of a variety of electrical components, such as anodes, grids, screens and the like, but behaves much like stainless steel with respect to machinability. It is not infrequent that the machined surfaces of such metal components are rough, torn and comparatively irregular. Often, this is prohibitive from the standpoint of manufacturing useable components with required electrical properties. For example, in the construction of a magnetron, the high potential and close spacing between the cathode assembly and anode gives rise to the phenomenon of sputtering in which are overs occur between rough surfaces of the cathode and anode. Apart from the gen eral problems of rough and irregular surfaces, it is excep tionally difficult to machine small components, such as the electrodes for a magnetron, to required dimensional tolerances. For example, it is known that the arc over voltage for a constant gap in a magnet'ron' is a function of the shape of the gap electrodes and that the rounding of the active or end faces of the gap electrodes will raise" tions, Rochelle salt solutions, and ammonia-containing solutions. The mentioned solutions as wellas'others'suggested for the electrolytic working of molybdenum have found' only limited applications in that the solutions-exhibit disadvant ges. Prominent among these disadvantages is the requirement that'the anodic treatment occur at elevated operating temperatures and with'relatively rigid temperature control; the leaving of a molybdic oxide film on the treated part which mustbe removedby subsequent processing; and the difficulty of handling and controllingperature with relatively non-critical temperature control,

such solutions, such as the need for inhibiting continued features, prominently the ability tooperate at room--tem--- present invention.

eifectiveness at a relatively rapid rate with little or no interfering films forming on the surfaces under treatment, and a relatively broad operating range of anode current densities.

Accordingly, it is a still further object of'the present invention to provide improved electrolytic solutions for polishing molybdenum parts which are not extremely critical with respect to temperature of operation, which facili tate electropolishirig at room temperatures, and which require little or no control over ambient temperature.

It is a still further object of the present invention to provide molybdenum electropolishing baths which leave treated metal surfaces bright, highly reflective and substantially free of films, such as not to require subsequent treatment.

It is a still further object of the present invention to provide improved electropolishing baths capable of operation at comparatively high current densities for achieving rapid levelling of surfaces being polished and having broad operating ranges of anode current densities.

We have found that an electropolishing bath consisting essentially of hydrochloric acid and sulfuric acid is highly suitable for anodic polishing of molybdenum. The presence of hydrochloric acid as an essential polishing ingredient in an electropolishing bath is contrary to the teachings of the printed literature which indicate that the addition of small amounts of hydrochloric acid is detrimental to polishing action.

Electropolishing' solutions inaccordance with the present invention and characterized by the presence of a relatively large amount of hydrochloric'acid with sulfuric acid exhibits a number of advantages, including the ability to electropolish molybdenum parts at room temperatures, the need for little or no temperature control, no appreciable filming or oxide coating. of the parts under treatment, a

broadrangeof anode current densities, and rapid levelling of parts under treatment. Additionally, the present electrolytic solutions facilitate polishing under more favorable different due to variations in current density over said irregular objects, the differences in such metal removal are relatively unimportant.

corned primarily with polishing grain irregularities and cracks on the surfaces of metal, the levelling of roughness 'due to mechanical working, and the rounding of sharp edges and corners.

The above brief description as well as further objects, advantages and features of the present'invention will be best appreciated by reference to the drawing in conjunction with the accompanyingdetailed description-cf presently preferred baths and treatment methods. 7

In Figure 1 of the drawing,.tnere is shown a ternary or triangular coordinate diagram illustrating the electropolisl iing region for the numerous solutions coming withinthe scope of the present invention; and,

In Figure 2' of the drawing, there, is shown a ternary diagram illustrating the lower operating limit ofcurrent densities for electro'polishin'g solutions according to the Referring now specifically to Figure l of the drawing, there is'illustr'ated a ternary diagram showing the relative proportions of hydrochloric acid and sulfuric acid for electropolishing with aqueous solutions in accordance with the present invention; The three coordinates'represent water which is presentinincreasing quantities along lines" Patented Apr. 1, 1958 The electropolishing action, herein involves rapid removal of the metal and is con parallel to and progressively removed from the zero percent water axis sulfuric acid which is present in increasing quantities along lines parallel to and progressively removed from the zero percent sulfuric acid axis 12; and hydrochloric acid which is present in increasing quantities along lines parallel to and progressively removed from the zero percent hydrochloric acid axis 14.

The solutions found suitable for electropolishing molybdenum are found in the region approximately enclosed by the solid lines in the triaxial diagram of Figure l, specifically by the solid curved line AB, and the solid straight line BA coinciding substantially with the zero percent water axis 10. The preferred compositions of molybdenum electro-polishing solutions lie within'the lesser region defined by the dot-dash curved line CD, and the solid straight line DC which coincides substantially with the zero percent water axis 10.

The approximate coordinates of the points defining the respective molybdenum electropolishing regions are as follows:

and 25 perand 81 perand 30 perand 74 per- Selection of a particular solution within the electropolishing regions depends upon practical considerations, such as the required width of the operating range and the power consumption. The wider the operating range, the more uniform will be the polishing action on recessed or irregularly shaped objects, while the lower the total amount of current required, the smaller the power source. The specific baths listed below are illustrative of the wide variety of formulations which may be used in accordance with the present invention. It is to be understood that no sharp transition occurs in the action of the bath as the outer limits of the defined electropolishing region are approached. Rather, the limits define a threshold region wherein the polishing action may be other than optimum and/ or the power consumption become prohibitive. Among the formulations recommended for electropolishing molybdenum are the following, which are listed in percentages by volume:

Of the above listed solutions, the 37.5 percent sulfuric, 37.5 percent hydrochloric and 25 percent water solution is found to be most suitable for electropolishing of molybdenum. These solutions operate at room temperature and with current densities of between 1500 to 20,000 ampercs per square foot with the work serving as the anode in conjunction with a cathode of an appropriate material, such as platinum or graphite. I

Because of the limited solubility of hydrochloric acid in sulfuric acid solutions, some of the illustrative baths are inherently unstable. Such inherent instability presents a problem of fuming and further cause slow changes in the composition of the electrolyte. Specifically, solutions 7 through 11 inclusive are unstable and fume with attendant industrial hazard, but otherwise are suitable for the desired polishing operation. Accordingly, it is desirable to employ solutions of a composition lying above the dashdash line 16 on the ternary diagram, which dash-dash line is the theoretical solubility line for hydrogen chloride gas in sulfuric acid.

The approximate coordinates of the intersection of the dash-dash theoretical solubility line 16 with the lines AB and CD which define the electropolishing regions are as follows:

E-34 percent water, 59 percent sulfuric acid, and '7 percent hydrochloric acid.

F-25 percent water, 45 percent sulfuric acid, and 30 percent hydrochloric acid.

G-7.5 percent water, 22.5 percent sulfuric acid, and

70 percent hydrochloric acid.

H--3 percent Water, 17 percent sulfuric acid, and 80 percent hydrochloric acid.

Referring now to Figure 2 there is shown a ternary diagram illustrating the lower operating limit of current densities for molybdenum electropolishing solutions according to the present invention. By conjoint refe1ence to Figures 1 and 2, the lower operating limit of current density can be ascertained for a given solution; addi tionally, the permissible operating range can be ascertained for said given solution. In the preferred electropolishing region of Figure 1 bounded by the dot'dasn line CD and the solid line DC (coinciding with the zero percent water axis) the operating range for current densities is in excess of 10,000 ampercs per square foot; whereas in the less preferred but still operative electropolzshing region defined by the solid lines AB and the dot-dash line CD and the segments AC, DB of the zero percent water axis, the operating range of current densities is between 5,000 and 10,000 ampercs per square foot.

The conjoint reading of the triaxial diagrams of Figures 1 and 2 to ascertain the lower operating limit of current density and the operating range is well understood, but the following is set forth for the purposes of illustration:

For the point on the diagram represented by the legend P, which indicates an electropolishing solution consisting essentially of 25 percent water, 35 percent sulfuric acid, and 40 percent hydrochloric acid, the permissible operating range of current densities is in excess of 10,000 ampercs per square foot. Upon reference to the cone sponding point P of Figure 2, it will be seen that the lower operating limit is less than 1200 ampercs per square foot. However, in that the transition of the various regions of Figure 2 are not abrupt and in view of the fact that point P lies exceptionally close to the region having a lower operating limit of between 1200 and 2500 amperes per square foot, it will be appreciated that the lower operating range should be selected relatively close to 1200 ampercs per square foot. Thus a permissible range of operation for the point P is between a lower value of approximately 1200 ampercs per square foot and the upper value of 12,000 ampercs per square foot. As is appreciated by inspection of the drawings the point P lies within the most preferred region of the present disclosure in that it is within a region having the highest range of operating current densities and in a region wherein there is no problem of fuming.

The accompanying diagram serves to indicate the relative proportions of hyrochloric acid, sulfuric acid and water which may be suitable for anodic polishing of molybdenum. However, the solution may include other ingredients such as other acids, metallic salts and contamination due to normal operation. For example, in making the electrolyte up for anodic polishing of molybdenum, the bath corresponding to the point P might be selected as lying within the most preferred area of the preferred compositions. During the continuous use of such bath in the electropolishing of molybdenum, the bath composition would necessarily change. The changing denum into the bath and the possibility of loss or addition of water content due ttr evaporation or absorption. Despite such changes in the composition as may occur during continued use, if the relative percentages of hydrochloric acid, sulfuric acid and water, expressed in percentages by volume, remain inthe preferred or less preferred regions of the diagram of Figure l, the bath will continue to operate satisfactorily.

The exceptionally broad range of operating current densities for the selected solution permits uniform polishing action on recessed or irregular shaped objects. The uniform polishing action is rapid due to the high operating current.

From the foregoing detailed disclosure, the several advantages of electropolishing solutions in accordance with the present invention should be fully appreciated. Among these advantages are the ability of the solutions to operate at room temperature, the non-critical nature of temperature control, the rapid actions of the solution which permit uniform polishing of irregular shaped objects, and the exceptionally broad and non-critical operating range of current densities which minimize need for rigid control over operating current and permits the electropolishing of highly irregular shaped objects.

Molybdenum objects polished in accordance with the present invention exhibit an enhanced and superior appearance under microscopic investigation, the electropolished molybdenum having an appearance very similar to clean mercury. Additionally the instant polishing action does not form oxide coatings, commonly referred to as moly-blue, which frequently appears as a flashwhen using hand polishing baths.

Although the foregoing description is based upon compositions for producing highly lustrous polishes on molybdenum, equally within the contemplation of the disclosure is the polishing of electrodeposited molybdenum, non ferrous molybdenum alloys, and other allied materials having molybdenum as an essential ingredient.

In the accompanying drawing, it will be appreciated that the preferred area bounded by the line AEHB and the line BA correspond approximately to that area in which the electropolishing solution contains between 5 and 80% by volume of hydrochloric acid, between 10 and 75% by volume of sulfuric acid, and not less than 55% by volume of water. This is a straight lineapproximation of the operative area, as specifically defined by the heavy lines. As previously pointed out herein the transition at the boundaries of the enclosed area are not sharp and there are threshold regions of operation contiguous to the boundaries which are operative for the purposes intendedhereiu.

Numerous modifications and substitutions in the present process will occur to those skilled in the art, and accordingly the appended claims should be given the latitude and interpretation consistent with the disclosure; at times certain features of the invention will be used without a corresponding use of other features.

What we claim is: I

1. The method of electropolishing a molybdenum part including the steps of making said part the anode in an aqueous electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid which are present in relative percentages by volume lying within the area defined approximately in the ternary diagram of Figure 1 by the solid line AEHB, and the solid line BAwhich coincides with the zero percent water axis.

2. The method of electropolishing a molybdenum part including the steps of making said part the anode in an aqueous electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid which are present in relative percentages by volume lying within the area defined approximately in the ternary diagram of Figure 1 by the solid line AEHB and the solid line BA which coincides with the zero percent water axis.

3. The method of electropolishing a molybdenum part including the steps of making said part the anode in an electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid which are present in relative percentages by volume lying within the area defined approximately in the ternary diagram of Figure 1 by the segment FG of the curved dot-dash line CFGD and the segment FG of the dash-dash line EFGH and extending above the dash-dash line EFGH and including the point P.

4. The method of anodically polishing an article having molybdenum surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients from 10 to 75% sulfuric acid, and from 5 to hydrochloric acid, said percentages being by volume and the relative percentages of said ingredients lying within the area defined in Figure l of the accompanying drawing by the lines AEHB and BA, and passing an electric current through said anode in an amount suificient to obtain an electropolishing action of said surfaces.

5. The method of anodically polishing an article having molybdenum surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients from 20 to 70% sulfuric acid, and from 25 to 75% hydrochloric acid, said percentages being by volume and the relative percentages of said ingredients lying within the area defined in Figure 1 of the accompanying drawing by the lines CFGD and DC, and passing an electric current through said anode in an amount sufficient to obtain an electropolishing action of said surfaces.

6. The method of anodically polishing an article having molybdenum surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients from 10 to 75 sulfuric acid, and from 5 to 80% hydrochloric acid, said percentages being by volume and the relative percentages of said ingredients lying within the area defined in Figure 1 of the accompanying drawing by the lines AEHB and BA, and passing an electric current through said anode in an amount sufficient to obtain an electropolishing action of said surfaces as'determined by Figure 2 of the accompanying drawing.

7. The method of anodically polishing an article having molybdenum surfaces including the steps of making said 7 article the anode in an electrolytic solution containing as essential ingredients from 20 to 70% sulfuric acid, and from 25 to 75 hydrochloric acid, said percentages being by volume and the relative percentages of said ingredients lying within the area defined in Figure 1 of the accompanying drawing by the linces CFGD and DC, and passing an electric current through said anode in an amount sufl'icient to obtain an electropolishing action of said molybdenum surfaces as determined by Figure 2 of the accompanying drawing.

8. The method of electropolishing a molybdenum part including the steps of making said part the anode in an aqueous electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid, said solution containing at least 10% by volume of sulfuric acid, 5% by volume of hydrochloric acid, and less than 55% by volume of Water.

9. The method of electropolishing a molybdenum part including the steps of making said part the anode in an aqueous electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid, said solution containing between 10 and 75% by volume of sulfuric acid, between 5 and 80% by volume of hydrochloric acid, and less than 55 by volume of water.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF ELECTROPOLISHING A MOLYBDENUM PART INCLUDING THE STEPS OF MAKING SAID PART THE ANODE IN AN AQUEOUS ELECTROLYTIC SOLUTION CONSISTING ESSENTIALLY OF SULFURIC ACID AND HYDROCHLORIC ACID WHICH ARE PRESENT IN RELATIVE PERCENTAGES BY VOLUME LYING WITHIN THE AREA DEFINED APPROXIMATELY IN THE TENARY DIAGRAM OF FIGURE 1 BY THE SOLID LINE AEHB, AND THE SOLID LINE BA WHICH COINCIDES WITH THE ZERO PERCENT WATER AXIS. 